Feline hyperthyroidism is the most common hormonal disorder in cats, particularly those aged ten years and older. It occurs when one or both thyroid lobes produce excessive amounts of thyroid hormone, driving the body’s metabolism into overdrive. Unlike hypothyroidism in dogs, feline hyperthyroidism is nearly always caused by a benign glandular abnormality rather than cancer.
Because the condition develops gradually, many owners mistake early signs for normal ageing. Early diagnosis and treatment are critical to protecting the heart, kidneys, and other organs from the sustained effects of excess thyroid hormone.
What Causes Feline Hyperthyroidism?
In more than 98% of cases, feline hyperthyroidism is caused by a benign adenomatous hyperplasia or adenoma — essentially, an overgrowth of thyroid tissue that functions autonomously without responding to normal hormonal feedback. Malignant thyroid carcinoma accounts for fewer than 2% of cases.
Risk factors and potential contributing causes include:
- Age: Cats over 10 are at greatest risk; the average age at diagnosis is 13.
- Diet: Diets high in iodine (especially certain commercial fish-flavoured foods) may play a role.
- Environmental chemicals: Polybrominated diphenyl ethers (PBDEs) found in flame retardants have been implicated in some studies.
- Breed: Siamese and Himalayan cats appear to have a lower risk than other breeds.
Common Symptoms of Hyperthyroidism in Cats
The classic presentation is a middle-aged to senior cat that is losing weight despite a ravenous appetite. Key clinical signs include:
- Weight loss with increased appetite (polyphagia): The accelerated metabolism burns calories faster than the cat can eat.
- Increased thirst and urination
- Hyperactivity and restlessness: Affected cats may seem younger but are actually overstimulated.
- Vomiting and diarrhoea: Rapid gastrointestinal motility causes digestive upset.
- Poor coat condition: Matting, oiliness, and unkempt appearance are common.
- Hypertension: High blood pressure is present in up to 87% of hyperthyroid cats.
- Heart murmur or gallop rhythm: Excess thyroid hormone induces hypertrophic cardiomyopathy in many cats.
- Vocalisation and behavioural changes
Some cats—particularly those with concurrent kidney disease—present atypically with lethargy and reduced appetite rather than the classic hyperactive picture.
Diagnosing Feline Hyperthyroidism
Diagnosis begins with a thorough physical examination. Vets often feel an enlarged thyroid gland (goiter) on neck palpation in 80–90% of affected cats. Confirmatory blood tests include:
- Total T4: An elevated result in a cat with compatible clinical signs is usually sufficient for diagnosis. Normal T4 in an otherwise suspicious cat warrants a free T4 or repeat test.
- Free T4 by equilibrium dialysis: More sensitive; detects occult hyperthyroidism when total T4 is borderline.
- Thyroid scintigraphy (nuclear scan): Gold standard for imaging thyroid tissue; determines if one or both lobes are affected.
Because hyperthyroidism can mask underlying kidney disease (by artificially maintaining good blood flow to kidneys), a full biochemistry panel and urinalysis should always accompany thyroid testing.
Treatment Options for Hyperthyroid Cats
Four main treatment strategies exist, each with distinct advantages:
- Oral medication (methimazole / carbimazole): Controls hormone production but does not cure the disease. Requires daily dosing for life. Useful for initial stabilisation and testing kidney function. Transdermal ear-flap gel is available for cats resistant to pilling.
- Radioactive iodine (I-131): The gold standard curative treatment. A single injection selectively destroys abnormal thyroid tissue while sparing normal cells. Cure rate exceeds 95%. Requires a brief hospitalisation period (typically 2–4 days) until radiation levels are safe.
- Surgical thyroidectomy: Removes the affected thyroid lobe(s). Effective but carries anaesthetic risks in older cats; risk of hypoparathyroidism if parathyroid glands are accidentally removed.
- Prescription iodine-restricted diet (Hill’s y/d): Limits iodine to reduce thyroid hormone production. Requires the cat to eat only this diet exclusively — difficult in multi-cat households.
Managing a Hyperthyroid Cat at Home
Regardless of treatment choice, regular monitoring is essential. Blood pressure checks should be performed at each visit. Kidney function must be watched closely, especially after starting treatment, as normalising thyroid levels can unmask previously hidden chronic kidney disease. Weight and muscle condition should be tracked monthly at home. Feeding a high-quality, high-protein diet supports muscle mass recovery. Stress reduction helps—avoid abrupt dietary changes, keep litter boxes clean, and provide calm resting spots.